 A 600-m-deep hole (Site PBS-2A) on the shelf within Prydz Bay aimed at coring the earliest
glacial and latest preglacial sediments identified by seismic mapping using the results of Leg
119.

 A 1020-m-deep hole (Site PBD-12B) on the continental rise that will provide a record of drift
formation comparable to shelf and slope records obtained during this leg and Leg 119.

 A 620-m-deep hole (Site PBF-6A) to provide a record of sedimentation in the Prydz Channel
Fan.

Site PBS-2AThe Mesozoic and Paleogene sediments beneath outer Prydz Bay form a series of seaward dipping
sequences (Figs. 6,14). Cooper et al. (1991a) recognized sequence PS.2A as Cenozoic glacial
sediments and the underlying Sequence PS.2B as preglacial, mostly nonmarine Mesozoic sediment.
Site 742 reached middle Eocene-lower Oligocene glacial sediments in Sequence PS.2A without
reaching the base of the sequence or nonglacial facies (Hambrey et al., 1991). Hambrey et al.
(1991) suggest that the base of the glacial interval was close because they interpreted preglacial
alluvial sediment mixed into the lowermost unit in the hole by subglacial deformation.

Proposed Site PBS-2A was chosen to recover core from the Cenozoic sediments below the horizon
reached in Site 742 into the top of Sequence PS.2B, the uppermost Mesozoic sediments identified
at Site 741. This site should provide an age for the arrival of glaciers in Prydz Bay, a record of
changes in depositional environments with the onset of glaciation, and an indication of changes in
biota. Site PBS-2A is the primary site because of its thin Quaternary diamict section and a slightly
thicker Paleogene section than at Site 742 and should be a more complete record. Site PBS-2A is
expected to encounter a thin Quaternary section of diamict and mud overlying an interval of upper
Eocene-lower Oligocene stratified diamictite with sandstone and mudstone interbeds similar to
sediments in Site 742 below 173 mbsf. Below the interval equivalent to the lower part of Site 742,
the lithologies are unknown, but the presence of reworked dinoflagelates and pebbles of
ferruginous marine marl, both of Eocene age, at ODP Sites 739 and 742 (Jenkins and Alibert, 1991;
Truswell, 1991) suggests a section containing shallow marine sediments. This site should reach
Cretaceous nonmarine sediments.

Site PBD-12BDrilling beneath the Antarctic Peninsula continental rise during Leg 178 showed that the drifts are
likely to provide fine-grained equivalents to continental slope and rise sediments. They should also
have microfossils for age estimates and interpretation of paleoceanography. Site PBD-12B was
selected to acquire a reasonably complete section of the drift record that includes Oligocene and
younger sediments. The site is located on the western flank of the Wild Drift (Fig. 14) where
Surface P3, which is thought to represent the arrival of grounded ice at the shelf edge during the late
Oligocene to early Miocene?, can be reached at about 1000 mbsf. It will also intersect Surface
PP.15 that marks the major late Miocene?-late Pliocene? change in glaciation on the shelf and the
base of the Prydz Channel Fan. The sediments expected at Site PBD-12B are fine grained
hemipelagic clays and distal turbidites with a higher biogenic component in intervals deposited in
interglacial periods compared to glacial intervals. Ice-rafted detritus should also be present.

Site PBF-6AThe slope site is aimed at the clinoforms of the Prydz Channel Trough Mouth Fan (Fig. 14).
Construction of the fan started in the late Miocene to middle Pliocene when the Lambert Glacier
formed a fast-flowing ice stream on the western side of Prydz Bay. The fan grew most during
episodes when the Lambert Glacier grounded at the shelf edge, delivering basal debris to the fan
apex. This material was then redistributed by sediment gravity flows and meltwater plumes.
Between such ice advances, the fan surface has received hemipelagic sediment. Thus, the alternation
of facies recovered from the fan should reflect the number of times the East Antarctic Ice sheet has
expanded to the shelf edge since late Miocene time. The age of inception of the trough mouth fan
will also indicate the time of a major change in glaciation style, probably related to a major increase
in the importance of coastal ice masses caused by cooling.

Gravity cores from the fan surface indicate that the section should consist of mud, turbidites, and
some debris flows deposited during glacial advances separated by finer, more biogenic intervals.
Microfossils present in gravity cores are diatoms and planktonic foraminifers. The optimum
position for drilling is in the mid fan, where mapped sequences are all present but not excessively
thick. Proposed Site PBF-6A is the primary site in this area.

Contingency Drilling
The development of climatic models and the prediction of future short-term climate changes
requires detailed records of past climates to determine what processes operated in the oceans and
atmosphere and to document the natural variability of global climate for the Holocene. Ice cores
have been the only source of such detailed information. Recent ODP drilling in Saanich Inlet, the
Santa Barbara Basin, Cariaco Basin, and Palmer Deep have yielded sedimentary sections with
annual resolution comparable to ice cores. Other sites around Antarctica have the potential to
provide similar sections. Having more than one sedimentary section from Antarctica would provide
the opportunity to study decadal and interannual variability of such climatic features as the
Circumpolar Wave and El Niño-Southern Oscillation. Deep valleys on the Mac Robertson Shelf
west of Prydz Bay (Fig. 11) contain thick Holocene ooze deposits that probably have such
resolution (Fig. 12). Therefore, if time permits, we will try to recover a high-resolution Holocene
environmental record by coring biogenic deposits on the outer Mac Robertson Shelf (proposed Site
PBS-3A).

The Holocene sequence in Iceberg Alley north of Mawson Station (Site PBS-3A) consists of a flat
lying fill within a U-shaped valley cut in older sediments and metamorphic basement (Harris and
O'Brien, 1996). Holocene sediments have been imaged by 3.5-kHz echo sounder and intermediate
resolution seismic and sampled by gravity coring (Fig. 12). The gravity cores recovered laminate
biosiliceous ooze showing laminations, some of which represent monospecific algal or resting
spore concentrations. Taylor (1999) interpreted down-core changes in diatom floras as reflecting
decadal-scale changes in productivity and water column characteristics during the Holocene.
Seismic data indicate that the ooze is about 50 m thick along the valley axis so that cores extending
through the full thickness of the deposit should have close to annual resolution. Site PBS-3A is the
primary contingency site, but there are a number of locations along the valley axis that contain
similar sediments (Fig. 12).